The Single Greatest Year.

[TobaccoKing4]

I think 1911 is the clear cut winner and that it isn't even close for the reasons already mentioned in this thread.

[darwinbulldog]

They're not all great, but 1910 does have the most sets listed in the OC chronological list -- E91C plus all of these:

[egri]

I'm going to say 1941. The last, and in my opinion, best Play Ball set was on the shelves, and kids could relatively easily obtain earlier cards by harassing their older brothers for their Goudeys and Diamond Stars, and fathers/uncles for their tobacco cards and CJs. Then while kids were collecting these cards brand-new, they were watching Joe DiMaggio's hitting streak and Ted Williams hit .406.

[Tao_Moko]

1909 was still in a decade of economic growth(4%), pre depressions, pre WWI and the birth of the most prolific set in baseball card history. Gets my vote.

[Joe_G.]

I'm biased, but 1887 is a strong contender . . .

Cigarette pack inserts skyrocket as the industry transitions from paper packs to cardboard slide & shell. Practically every subject matter would follow including baseball.

1887 Greatness
Old Judge, Gypsy Queen, Buchner Gold Coin, K-Bats (players & team cards), Four Base Hits, as well as the first baseball currency issues.

1888 - Strong Follow-up
A&G Champions, Goodwin Champions, Kimball Champions, Yum Yums, four different SF Hess baseball issues, OJs (both cigarette cards & cabinets), more currency, and the first gum issues courtesy G&B and HD Smith (Y95 Scrapps).

[brianp-beme]

1910 is my vote, but the whole stretch from 1909 through 1912 includes a huge chunk of the Deadball era cards that a lot of us collect, and blows away all other four year stretches hands down. A beater collector like myself thanks all the producers of the various E and T cards (definitely the heyday for these ACC classifications) for their overwhelming prolific-ness during this time.

Below I have depicted just a small percentage of the sets that were available during that 4 year period.

Brian

[SD]

Goudey, diamond stars, delong, George miller, & orbits. All beautiful sets